1. Field of the Invention
The present invention relates to a multilevel Frequency Shift Keying (FSK) modulator, and particularly to a multilevel FSK modulator applicable to a transmitter for a paging system.
2. Description of the Related Art
As is well known, the FSK (Frequency Shift Keying) format has the advantages that modulated signals transmitted according to this format are of constant envelope wave form, and consequently, there is little level fluctuation or noise, and that modulating and demodulating circuits can be easily constructed. Modulation according to the FSK format is carried out either by switching n number of oscillators corresponding to an n-level input signal, or by controlling the voltage-controlled oscillator (VCO) according to the control voltage signal corresponding to the input signal. In the latter case, the modulating circuit includes a phase-locked loop (PLL) in order to stabilize the frequency.
The European Telecommunication Standards Institute (ETSI) established the standards for the European Radio Message System (ERMES) as a new paging system in 1991. According to these standards, it is established that the term "symbol" is defined as
Two bits of information which are the basic unit of information on the air interface. It corresponds to one of the four modulation levels specified in subclause 9.3.1 of ETS 300 133-4 (4). [cf. Paging System: European Radio Message System (ERMES) , Part 6: Base station conformance specification, 3 Definition]; PA0 n=67 when the symbol is "10", PA0 n=65 when the symbol is "11", PA0 n=63 when the symbol is "01", and PA0 n=61 when the symbol is "00". PA0 f1=67 MHz when the symbol is "10" PA0 f1=65 MHz when the symbol is "11" PA0 f1=63 MHz when the symbol is "01" PA0 f1=61 MHz when the symbol is "00" PA0 f2=100 kHz+4687.5 Hz when the symbol is "10" PA0 f2=100 kHz+1562.5 Hz when the symbol is "11" PA0 f2=100 kHz-1562.5 Hz when the symbol is "01" PA0 f2=100 kHz-4687.5 Hz when the symbol is "00"
that four-level pulse amplitude modulated FM (4-PAM/FM) is used as the modulation format [cf. ibid. 6.2 Modulation]; and that a rise (or fall) time of 88.mu. sec for the frequency transition between two successive symbols is the standard of the transient response characteristic of the modulator [cf. ibid. 6.2.1 Symbol Transition Shaping].
FIG. 1 is a block diagram showing an example from the prior art of a multilevel FSK modulator applicable to the above-described paging system. This modulator carries out 4-level FSK modulation in response to an input data signal D.sub.i composed of a 2-bit symbol.
The voltage-controlled oscillator 1 is controlled by a phase-locked feedback circuit made up of a programmable frequency divider 2, a phase comparator 3, a loop filter 4, and a standard oscillator 5 and generates a 4-level FSK modulated signal S1. A frequency divider 7 frequency-divides the 4-level FSK modulated signal S1 and converts it to a signal S2 having a prescribed frequency shift. A mixer 8 mixes a local signal S3 supplied from a local oscillator 9 with signal S2 and converts the signal S2 so that it has a prescribed center frequency. An output filter 10 is made up of a filter circuit having a linear-phase characteristic and a band pass characteristic that passes a desired frequency band in order to prevent generation of high frequency components due to frequency transition and eliminate unnecessary frequency components, and transmits an output signal S.sub.0.
In operation, a data converter 6 receives input data Di, sets a scale factor n corresponding to each 2-bit symbol, produces a signal Cn indicating the scale factor and delivers it to the programmable frequency divider 2. The voltage-controlled oscillator 1 changes the oscillation frequency in response to the control voltage Cv and outputs signal S1. The programmable frequency divider 2 frequency-divides the signal S1 according to the scale factor n designated by signal Cn and sends the output signals Sn to the phase comparator 3. The phase comparator 3 compares the phases of standard signal Ss generated by the standard oscillator 5 and frequency-divided signal Sn and produces, by way of the loop filter 4, a voltage corresponding to the phase difference as the control voltage Cv.
As an example, it is assumed that the range of oscillation frequency of the voltage-controlled oscillator 1 is 61 MHz-67 MHz and the scale factor set by the data converter 6 is
Furthermore, by making the frequency of standard signal Ss 1 MHz, the frequency f1 of signal S1 can be controlled as follows:
In other words, a 4-level FSK signal having frequency that shifts corresponding to four symbols can be produced. In addition, the scale factor of the divider 7 is set in order to achieve a fixed frequency shift. For example, if the scale factor is 640, the frequency f2 of signal S2 is as follows:
In the above-described multilevel FSK modulator of the prior art, the 4-level FSK signals are produced by controlling, in accordance with input data Di, the scale factor to be supplied to the programmable frequency divider provided in the phase-locked loop. Accordingly, the transient response for frequency transition of the FSK signal due to a change in the input symbol (known as symbol transition shaping by ETSI) is determined by the response (step response) characteristic of the phase-locked loop. As is easily understood from automatic control theory, it is no simple matter to set the parameters of the loop circuit such that the loop circuit will have both a desired transient response characteristic and stable characteristics free of overcontrol or oscillating behavior. This point has been a key problem with FSK modulators to date. An additional problem has been that, when revising the transient response characteristic of a PLL, it is necessary to revise the phase characteristic of the output filter provided on the output terminal side as well, which involves redesigning and readjusting of the output filter.
The multilevel FSK modulator described above is a typical example of an FSK modulator that employs a PLL, but an example of an FSK modulator that does not employ a PLL is disclosed in Japanese Patent Laid-open No. 135555/82. The FSK modulator that uses a PLL and an FSK modulator described below that does not use a PLL will hereinafter be referred to as the first and second FSK modulators of the prior art, respectively.
The second FSK modulator of the prior art is intended to narrow the bandwidth of the spectrum of the output modulated signal by smoothing the phase change at the transition point from mark to space of an input data signal (a modulating signal). To this end, when altering the scale factor at the transition between mark and space, subdivided intervals are provided that take intermediate values of the scale factor between the marking and spacing intervals, and the frequency transition is made from the mark to space frequencies through subfrequencies corresponding to the subintervals, or vice versa.
Because this FSK modulator does not employ a PLL, difficulty is to be expected in obtaining a stable center frequency or a stable frequency shift, and there are also the following additional problems: because the frequency of the output modulated signal is produced by dividing the standard frequency by a programmable frequency divider, the standard frequency must be a common multiple of the output frequency. However, in the above-mentioned ERMES paging system in which the frequency shift of an output signal is prescribed, for example, to be 4.6875 kHz, 1.5625 kHz, -1.5625 kHz, -4.6875 kHz (i.e., if the center frequency is 100 kHz, the output frequency is (100+4.6875) kHz, (100+1.5625) kHz, (100-1.5625) kHz, (100-4.6875) kHz), it is difficult to set the standard frequency at a common multiple of these output frequencies; moreover, the second FSK modulator of the prior art is not provided with a means to regulate the response characteristic for the transition between the mark and intermediate frequencies and the transition between the intermediate and space frequencies. Accordingly, in a case in which symbol transition shaping for an ERMES paging system is prescribed as (88.+-.2).mu. sec., it is difficult for the second FSK modulator of the prior art to satisfy the required conditions.